Fix layout problems in Firefox 49+

Firefox 49 introduces support for non-standard `-webkit`-prefixed CSS
properties. This breaks Sencha Touchs layouting CSS.

https://hacks.mozilla.org/2016/09/firefox-49-fixes-sites-designed-with-webkit-in-mind-and-more/

Additional CSS has been introduced for Firefox to ignore problematic
`-webkit` properties.

src/main/webapp/resources/sass/app/_bugfixes.scss

@@ -65,3 +65,20 @@ body.x-android.splashscreenClosed {
 body.x-windows {
 	touch-action: none !important;
 }
+
+body.x-firefox {
+	/* Override bad CSS code of Sencha Touch which breaks layout in Firefox 49+.
+	 * See https://hacks.mozilla.org/2016/09/firefox-49-fixes-sites-designed-with-webkit-in-mind-and-more/
+	 */
+	.x-button, .x-layout-box {
+		display: flex;
+	}
+
+	.x-layout-box > .x-layout-box-item {
+		display: flex !important;
+	}
+
+	.x-stretched.x-container > .x-inner, .x-stretched.x-container > .x-body, .x-stretched.x-container > .x-body > .x-inner {
+		display: flex !important;
+	}
+}